Specifications

42 Rockwell Automation Publication AG-SG001G-EN-P - April 2015
Chapter 3 Choosing Data Communication Equipment
Choosing a Telephone
Modem
Modems convert digital information from a programmable controller or
computer to an analog signal that is compatible with the communication media
being used. The signal is then transported to the receiving modem, which
converts the analog signal back into a digital one.
In the illustration below, digital data from each DTE is converted to an analog
signal for transmission over the communication media.
Two modem technology standards exist to make certain that modems developed
by different manufactures are compatible.
In most cases, the two modem types are not compatible. Keep this in mind when
choosing modems for stations that are being added to or are retrofits for an
existing installation. Compatibility charts exist. Consult a modem supplier for
more information.
Use the selections that you recorded from the previous section, ‘Choosing a
Telemetry Network’, to answer these questions:
• What type of links are you using to transmit data (for example, PSTN,
private leased line, or radio)?
• What transmission modes are you using (half-duplex, full-duplex)?
• What are your network topologies (point-to-point, point-to-multipoint)?
• Are you using 2-wire or 4-wire lines?
Once you know the type of modem, use these criteria to help you choose
appropriate models:
• Required data communication rate.
DTE DTE
DCEDCE
Type Description
Bell The Bell standard was the predominant standard in the
United States until the break-up of AT&T in the USA.
Consultive Committee for
International Telephone and
Telegraph (CCITT)
The CCITT standard is the international standard that
is now becoming the standard for the USA. Most
modems now conform to one or more of the CCITT
standards, such as V.32, V.32bis, and V.22.